1. Field of the Invention
The present invention relates to a developing device using a two-component developer composed of a carrier and a toner, and an electrophotographic apparatus using the developing device.
2. Description of the Related Art
Electrophotographic apparatuses such as a copying machine, a printer, etc. using electrophotography have become essential to current business. This type electrophotographic apparatus operates as follows. After a photoconductor is electrically charged, the photoconductor is exposed to light in accordance with image data so that an electric charge distribution corresponding to an image pattern is formed on the photoconductor. When toner is developed in accordance with the electric charge distribution, a toner image first appears as a visible image. The toner image is then transferred onto a sheet of paper and thermally fixed on the sheet of paper. Thus, the formation of an image is completed.
In this development, there is used a developing method using a two-component developer as a mixture of a toner of colored particles of resin powder having a particle size of from 5 to 10 μm and a carrier of magnetic particles of ferrite, magnetite or iron powder having a mean particle size of from 30 to 100 μm, or a developing method using a one-component developer having magnetic powder in a toner without using a carrier. In the electrophotographic apparatus using a two-component developer, the developer is carried to a space (developing portion) where the photoconductor faces a developing roll having a magnet, by the developing roll so that toner is developed on the photoconductor. As the developing roll, there is commonly used a developing roll having a structure in which a magnet roll is provided on the inside of a cylindrical sleeve roll. On the other hand, an electrophotographic apparatus using a plurality of such developing rolls is known. Achievement of high printing quality is required of the electrophotographic apparatus. Therefore, various measures have been tried to the developer and the developing device.
For example, there is known a technique in which the resistivity of the carrier and the carrier loading rate in the developing portion are limited so that the carrier resistance and the loading rate are set to be in proper ranges respectively by an AC developing bias to thereby prevent beads carry over (a phenomenon that the carrier is deposited on the photoconductor) and white dotting in a solid image (e.g. Japanese Published examined application Hei. 7-062779).
There is also known a technique in which the carrier resistance, the frequency of the AC developing bias and the volume-average particle size of the toner are combined properly to thereby obtain an image free from fogging and rear end missing and good in graininess (e.g. Japanese Patent No. 2768078).
There is known a further technique in which the carrier particle size is reduced to bring the particle size distribution and the specific surface area based on an air permeation method to proper ranges respectively to thereby prevent lowering of image density and blurring at the time of continuous copying of a large-area image (e.g. Japanese Patent 3029180).
There is known a further technique in which a developing device having a combination of two developing rolls different in the direction of movement relative to a surface of the photoconductor is used to prevent rear end missing of an image (e.g. JP-A-10-232562).
There is known a further technique in which a ferrite carrier having a core material of iron oxide having magnesium oxide and/or manganese oxide as a subsidiary component is used in a high-speed printer using a plurality of developing rolls to achieve a two-component developing method/developer of high quality and long life (e.g. Japanese Patent 3418604).
These background-art techniques are however insufficient to reduce the size of the developing device in the high-speed printing electrophotographic apparatus. For example, there are problems that sufficient print density cannot be obtained because the amount of toner carried to the developing portion is reduced in order to suppress the carrier loading rate in the developing portion, that rear end missing occurs at high-speed printing, that high image quality cannot be achieved though rear end missing is prevented, and that rubbing irregularity of the carrier appears in the image when the carrier used has a large mean particle size of 110 μm. If the size of the developing device is increased to keep the print density high, it is difficult to handle such a large-size developing device because the large-size developing device can be hardly held by human hands.